High-energy cosmic rays are flying through space and bombarding the Earth every day.

We just can’t see them. They’re also to blame for a strange side-effect of space travel: mysterious flashes of light that astronauts see out of the corners of their eyes.

These so-called cosmic ray visual phenomena have been reported enough times that scientists are trying to research what’s going on with astronaut eyeballs. But we still don’t really understand why these flashes happen, or how exactly cosmic rays damage eyes and nervous systems, so we can better protect our astronauts. Despite the name, cosmic rays aren’t really rays of light at all.

They’re extremely high-energy subatomic particles, mostly protons or atomic nuclei, that are being flung through space and bombarding the Earth. We’re not exactly sure where cosmic rays come from. One main theory is that explosive supernovae outside of our solar system basically act like huge, natural particle accelerators and generate them.

Now, Earth’s magnetic field is pretty good at protecting us surface-dwellers from cosmic rays and other charged particles, like the ones from solar wind, by deflecting them. Lots of these energetic particles are trapped in donut-shaped bands around the Earth called the Van Allen belts, which are like radiation-filled minefields for anything floating around in space. We try to make sure our Earth-orbiting satellites and space stations are nestled in the gaps between Van Allen belts, to keep the people and equipment safe.

But there are certain regions, like one called the South Atlantic Anomaly, that orbiting spacecrafts end up passing through. So unfortunately they get bombarded with all those high-energy particles. All that radiation isn’t great for humans, and is one of the biggest dangers of spending a lot of time in space, outside of the protective blanket of Earth’s magnetic field.

That’s why organizations like NASA study astronaut health so carefully. Long-term exposure might increase astronauts’ risk of getting cancer, and it has weird, harmful effects on their eyes and nervous systems, like increasing the likelihood of cataracts many years later. Sometimes, when astronauts are exposed to cosmic rays, they even start seeing flashes of light every few minutes, which we call cosmic ray visual phenomena.

Some space travelers are especially sensitive to the flashes, while others never report noticing them at all. The Apollo astronauts who first described them said that they mostly look like spots, though sometimes they appear to be streaks and clouds. They were bright enough that they even kept some astronauts awake at night.

By taking a close look at the Apollo astronauts’ helmets, NASA was able to see tracks left by subatomic particles passing through the plastic. And in the 1970s, some ground-based scientists conducted some maybe-not-super-safe experiments. They had people look at neutron beams in dark rooms, which caused similar flashes to appear in their vision.

Some more modern astronauts are outfitted with special helmets with cosmic ray detectors. The astronauts keep track of when they see a flash, to collect data on what types and energy levels of particles seem most likely to cause them, and how often they appear in different parts of space. So we’re pretty sure that cosmic rays are the cause of the lights, and researchers hope to use these data to come up with better ways to protect astronauts from cosmic radiation.

But there’s still a big mystery: we don’t really know what’s happening inside astronauts’ eyeballs when these flashes occur. One possible culprit is Cherenkov radiation, which causes the blue glow you might have seen in photos of nuclear reactors. Cherenkov radiation happens when really energetic particles pass through a liquid, faster than light can travel through it.

Those particles can interact with some of the liquid molecules, which respond by emitting some photons of light -- the glowing Cherenkov radiation. Cosmic rays passing through eyeball fluid could be generating some Cherenkov radiation, which astronauts see as spots and flashes. Another possibility is that radiation from cosmic rays could interact weirdly with the retina, the tissue at the back of your eye that normally detects visual light.

Or, cosmic rays could bypass normal vision completely and pass right through the eyeball, directly stimulating the optic nerve or even the brain’s visual cortex. Scientists still aren’t totally sure what the right explanation is, since it’s hard to do research on the eyeballs of busy astronauts in space. The flashes could even have multiple causes, which would explain why they don’t all look the same.

Cosmic ray visual phenomena are ultimately a pretty minor issue compared to being smashed by meteors or fried by solar flares. But they are a possible indication of damage to astronauts’ eyes or even brains. And cosmic radiation is one huge obstacle have to contend with if we ever do leave our home planet behind to travel the galaxy.

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